Motion Vector Derivation Method, Moving Picture Coding Method and Moving Picture Decoding Method

1. An image coding method for coding a current block in a current picture, the method comprising: obtaining a reference motion vector of a reference block, the reference motion vector being used for deriving a motion vector of the current block to be coded; calculating a first parameter corresponding to a difference between a display order of a picture including a reference block and a display order of a reference picture of the reference block, wherein said reference block is motion-compensated using the reference motion vector, and said reference picture is referred to by the reference motion vector; calculating a second parameter corresponding to a difference between a display order of a current picture and a display order of the reference picture, wherein said current picture is a picture including the current block; judging if (i) the reference motion vector of the reference block refers to a picture having a display order located after a display order of a picture including the reference block and (ii) the first parameter is a negative value within a predetermined range; generating a multiplier parameter corresponding to the first parameter, the multiplier parameter being used for changing a division operation by the first parameter into a multiplication operation by the multiplier parameter; deriving the motion vector of the current block by scaling the reference motion vector based on a multiplication of a multiplier parameter corresponding to a predetermined negative value and the second parameter, when the first parameter is a negative value out of the predetermined range as a result of said judging, and by scaling the reference motion vector based on a multiplication of a multiplier parameter corresponding to the first parameter and the second parameter, when the first parameter is a negative value within the predetermined range as a result of said judging; generating a motion compensated image of the current block using the derived motion vector and a reference picture corresponding to the derived motion vector; and coding a difference image between the current block and the motion compensated image of the current block.

2. An image coding apparatus which codes a current block in a current picture, the apparatus comprising: a unit operable to obtain a reference motion vector of a reference block, the reference motion vector being used for deriving a motion vector of the current block to be coded; a unit operable to calculate a first parameter corresponding to a difference between a display order of a picture including a reference block and a display order of a reference picture of the reference block, wherein said reference block is motion-compensated using the reference motion vector, and said reference picture is referred to by the reference motion vector; a unit operable to calculate a second parameter corresponding to a difference between a display order of a current picture and a display order of the reference picture, wherein said current picture is a picture including the current block; a judging unit operable to judge if (i) the reference motion vector of the reference block refers to a picture having a display order located after a display order of a picture including the reference block and (ii) the first parameter is a negative value within a predetermined range; a multiplier parameter generating unit operable to generate a multiplier parameter corresponding to the first parameter, the multiplier parameter being used for changing a division operation by the first parameter into a multiplication operation by the multiplier parameter; a motion vector derivation unit operable to derive the motion vector of the current block by scaling the reference motion vector based on a multiplication of a multiplier parameter corresponding to a predetermined negative value and the second parameter, when it is judged by said judging unit that the first parameter is a negative value out of the predetermined range, and by scaling the reference motion vector based on a multiplication of a multiplier parameter corresponding to the first parameter and the second parameter, when it is judged by said judging unit that the first parameter is a negative value within the predetermined range; a unit operable to generate a motion compensated image of the current block using the derived motion vector and a reference picture corresponding to the derived motion vector; and a unit operable to code a difference image between the current block and the motion compensated image of the current block.